bjt input buffers

[jogina111]

I learned that bjt buffers have low input impedance compared to jfets and opamps..is it possible that 2 bjt buffer stages can make a good preamp? I have an lm358 opamp but they say 358 chips are unstable...

[Cliff Schecht]

Without going into extreme detail, a Darlington pair and/or bootstrapping can be used to increase the input impedance of a BJT input stage. There are many other techniques but these are common ones. Here's some articles:

http://en.wikipedia.org/wiki/Darlington_transistor
http://en.wikipedia.org/wiki/Bootstrapping_(electronics)

Also the LM358 is internally compensated for stability even as a unity gain follower (the toughest condition for stability). Unless the circuit you are using somehow adds instability (i.e. driving a very large capacitance connected to ground which can degrade stability of a two-stage amplifier), I disagree with the notion that this device is unstable!

[jogina111]

how many c9014 would it take to match a jfet?

[Cliff Schecht]

You won't match a JFET. The gate of a JFET draws almost no current because it is physically isolated from the drain and source whereas a BJT draws a relatively significant amount of base current (dependent on Beta) because it is physically connected to the emitter through a diode. Input resistance on a JFET is typically in the Teraohms whereas a BJT is all over the place depending on biasing (lower base current is equivalent to a higher input impedance).

With that said, we typically don't use JFET amplifiers with a super high input impedance. If the source feeding the JFET amplifier is also high impedance then the typical auto-biasing JFET circuit has no reference for the gate current to flow and develop bias. Something like a 1M resistor to ground is typical to set the input impedance to a known value.

The bootstrapping method can be used to get a BJT to have a very high input impedance, however this usually is not necessary with modern BJT's to achieve acceptable performance. It's an older method used to improve the input impedance of the low Beta BJTs at the cost of stability. The Darlington pair can also attain much higher input impedance as the Beta of each device will be multiplied (thus reduce base current and increasing input impedance). However, this topology has an input swing that is now reduced to at minimum two diode drops above ground. Not good for handling large signals.

With all of this said, may I ask what your application is? It sounds like you are going to unnecessary measures to do what is nowadays considered trivial with general purpose op amps..

[jogina111]

I wanna put an input buffer to a td2003 amp a friend lend me but all I got is a bunch of bjt's like c9014, 1300 and 8550..

[gritz]

^^^What Cliff said^^^

If you're interfacing to a guitar then a bjt transisor buffer is fine, speaking pragmatically.

A fet transistor does have a higher input impedance than a bjt, but it's all relative. A typical bjt input buffer with a hfe (current gain) of say 300 and an emitter resistor of 10k represents an imput impedance of 3meg in parallel with the (high) value of your bias resistor. As Cliff says a bias resistor is required with a fet buffer also. In a guitar scenario the value of the input impedance (typically in the range of 300k - 1m for buffered pedals) will be matched by the capacitance of a typical guitar cable at somewhere vaguely in the region of 300 - 1000Hz, so an infinite pedal input impedance doesn't really gain anything as your guitar's pickup will "see" a lowering impedance (and so a higher load) above the midrange, regardless of the buffer's impedance...

[Gus]

There is the Sziklai pair to try instead of the Darlington
sim in the thread http://www.diystompboxes.com/smfforum/index.php?topic=98244.0

What is the input resistance of the amp?  This is to know what kind of buffer output drive is needed.
What will be before the buffer?  a guitar or an effect or...?

[Gus]

Some screenshots from a sim
A buffer circuit that you can find in effects.  470k, 510k base to Vref with a 10K emitter resistor
If you use a higher Hfe transistor you might want to adjust R4